The wood thrush weighs only 40 to 50 grams, or less than two ounces. It’s smaller than a robin but bigger than a goldfinch.

The tricky part isn’t tying on the little backpacks. They have two straps, just like a human’s, though these loop around the birds’ legs and the pack itself sits low, like a fanny pack. Feathers hide most of it and it doesn’t impede flying, or the bird’s love life.York University

OTTAWA — Canada’s millions of migrating songbirds are creatures of habit, a point of character that could cause trouble if they need to adapt to a changing climate.

This was the important finding gleaned from years of studies of the wood thrush that showed birds lucky enough to survive their first year (half don’t) fly north on almost exactly the same date each spring.

The route may change: from Central America through Louisiana one year and up through Florida the next. The timing, though, is rigid.

“Birds that leave early one year leave early the next,” said Bridget Stutchbury of York University, a biology professor who led a team of student researchers. Those that depart late are equally late each year.

This is considered surprising because conditions of weather and food supply change radically from year to year.

Spring migration is a gamble involving life, death and sex. Birds that leave early get the best territory in Canada and have more success attracting a mate and raising young. But they also risk freezing or starving if the spring weather turns bad.

“It suggests that these animals have an internal schedule of migration,” the biologist said.

Autumn migrations are a bit more flexible. But they’re less urgent, since the birds arriving in Central America aren’t under pressure to nest and find a mate.

Having a rigid schedule may hurt the birds if our climate changes and the birds have trouble adjusting their schedule, she suggests. She suspects warmer weather would favour birds that fly north early, and evolution could eventually weed out the others.

The wood thrush weighs only 40 to 50 grams, or less than two ounces. It’s smaller than a robin but bigger than a goldfinch.

The York University team used tiny backpacks (geolocators in formal parlance) to collect data on what routes the birds have taken during spring and fall migrations, and when they flew.

The tricky part isn’t tying on the little backpacks. They have two straps, just like a human’s, though these loop around the birds’ legs and the pack itself sits low, like a fanny pack. Feathers hide most of it and it doesn’t impede flying, or the bird’s love life.

But the backpack doesn’t transmit, so Stutchbury’s students have to find birds when they return the following year. They have retrieved wood thrushes all the way from Kingston to Pennsylvania.

Their study is published in a research journal called PLoS ONE.

Other findings:

Males fly faster. They also tend to come back to the same territory more reliably than females, making them easier to retrieve.

First-time flyers are slower than birds that have made the trip once already.

Most birds spend the same number of days migrating each year even if bad weather over the Gulf of Mexico forces them far off course. “It’s almost as though they can make up (for lost time) en route,” Stutchbury says.